Why can I no longer scroll threads in Chatgpt?

Nothing on Mozilla side. Since you're running firefox.com/firefox/115.38.0/releasenotes/ from July 21 (and the last firefox.com/firefox/115.40.0/releasenotes/ is from September 1 anyways) what you see is a site change on openAI side — you might need to ask their support for any recent changes.

Naturally I did that. It was working fine until 2 days ago. Chatgpt does not know what to do. I just installed an addon called tampermonkey to see if that will work. If not, I can no longer use my Imac which is sad because it's the EASIEST OS and all my software that I need works fine. I am 74. Been on a Mac since 1988. I do have newer macs but hate the new OS.

I can understand that & have my fair share of classic Macs running a few different OS versions.

On macOS 10.14 you will have about six more months of Firefox ESR security updates, however site operators may elect not to support rendering engines of this vintage. To run anything newer, incl. the current releases, you'd need at least macOS 10.15 — if your Mac can't run it officially, I'd recommend searching for "unsupported macs" to see how to upgrade even beyond what the factory permits.

As for your current breakage, I assume you've tried troubleshooting using new Private Window and restarting into Troubleshoot Mode (from Help menu) to eliminate any potential session flaws, site data causing that, or perhaps addons interfering?